There are many applications for positioning systems having accurate controllable motor drives. One common application is in X-Y plotters. The typical plotted employs an X-axis friction drive mechanism for the chart or like media, and a Y-axis direct drive for the pen, the combination of movements allowing the plotter to produce virtually any kind of line drawing on the chart.
One of the essential conditions for the drive mechanisms in such apparatus is that there should be minimum slippage and backlash in the system. This is required because the execution of a line drawing by the plotter requires that the chart continuously move back and forth in the X-direction, and it is important that the X-axis drive mechanism be capable of repeatedly positioning the chart to very close tolerances at the selected positions along the X-axis. The same requirement, of course, exists for the Y-axis drive mechanism for the pen.
These problems are exacerbated when a belt-pulley system is employed in the drive mechanism. The latter system is preferred mainly because it reduces the cost of the drive mechanism, and it somewhat simplifies the location of parts in a housing that historically keeps getting smaller and smaller. But, to satisfy the requirements indicated above with a belt-pulley drive system requires a mechanism not only for providing the proper tension on the belt, but also for maintaining the belt tension at the correct value and substantially constant during the lifetime of the product.
In one known system, a spring-leaded, cam-driven, idler roller operates against the outer surface of the belt producing a reverse wrap which provides the required tension. However, this arrangement offers certain disadvantages.